Method of forming carbon resistor films



Dec. 17, 1968 R. L. coRMlA 3,416,959

METHOD OF FORMING CARBON RESISTOR FILMS Filed Feb. 26, 1965 /nyemor Roban* L. Cor/nia,

His Alorne y.

United States Patent O 3,416,959 METHOD F FURMING CARBON RESISTGR FILMS Robert L. Cormia, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed Feb. 26, 1965, Ser. No. 435,546 4 Claims. (Cl. 117-213) ABSTRACT 0F THE DISCLOSURE Carbon lms having a resistance of 5000 ohms per square with less than .3% change in permanent resistance after a minute heat treatment at 400 C. are formed by successive depositions in a pressure range between 1 109 to 5x10*5 millimeters of mercury of a silicon monoxide layer, a carbon layer, metallic contacts, and a silicon monoxide overcoating atop a non-conductive substrate. The substrate preferably is heated to a temperature between 200 to 600 C. during the depositions and all depositions are eifectuated Without a breaking of the vacuum of the chamber. Resistors of a desired ohmic value are obtained either by a careful monitoring of the deposition rate or by a selective oxidation of a portion of the carbon lm prior to the deposition of the silicon monoxide overlayer.

This invention relates to methods of making electrical resistors, and more particularly to resistors methods of making evaporated carbon resistors.

Carbon resistors which have been prepared by painting a carbon film on a substrate have posed various problems in the employment of these resistors. Such resistors have exhibited a .poor adherence of the carbon film to the substrate. A serious problem is also encountered in that the resistance of such a resistor changes substantially upon exposure to an elevated temperature.

Thus, it would be `desirable to provide a method of forming a carbon resistor which exhibits good adherence to its substrate, stability during operation, and a minimum amount of change in resistance due to the elevation of the temperature of the resistor. The present invention therefore is directed to methods of making evaporated carbon lilm resistors wherein the problem of instability, poor adherence and changing resistance dfue to temperature change are substantially reduced or eliminated.

It is an object of my invention to provide a method of forming a carbon resistor which is operable over a wide temperature range with a minimum change in resistance.

It is another object of my invention to provide a method of forming a resistor wherein the carbon iilm exhibits good adherence to its substrate.

It is Ianother object of my invention to provide a method of forming a carbon resistor which is stable during operation.

It is a further object of my invention to provide a method of forming a carbon resistor having a predetermined resistance value.

In carrying out my invention in one form, an electrical resistor is formed which comprises an electrically nonconductive substrate, a layer of silicon monoxide adhering firmly to the substrate, a layer of carbon with a predetermined resistance value adhering to the silicon monoxide layer, a pair of spaced apart electrically conductive electrodes adhering to the carbon layer, and a second layer of silicon monoxide adhering to the carbon layer.

These and various other objects, features and advantages of the invention will be better understood from the Patented Dec. 17, 1968 ice following description taken in connection with the ac4 companying drawing in which:

FIGURE l of the ydrawing is a perspective view of z resistor formed by the method of my invention;

FIGURE 2 is a sectional view of the resistor showt in FIGURE l of the drawing; and

FIGURE 3 is a perspective view of apparatus for forming an electrical resistor in accordance with my invention In FIGURES 1 and 2 of the drawing, there is showr generally at 10 a resistor formed by the method of m3 invention. This resistor comprises a substrate 11 of ar electrically non-conductive material such as slide glass Pyrex glass, or a glazed ceramic, which substrate providef a smooth surface for subsequent condensation of materialf thereon. A layer 12 of silicon monoxide is shown adhering firmly to substrate 11, A layer 13 of evaporatecl carbon, which has a predetermined resistance value, adheres to silicon monoxide layer 12. A pair of spacerA apart electrically conductive electrodes 14 of aluminurr adhere to carbon layer 13. A second layer 15 of silicor monoxide adheres to carbon layer 13 and is shown cover ing a portion of electrodes 14.

While the substrate 11 is described as being composer of slide glass, Pyrex glass or a glazed ceramic, othel substrates, which are electrically non-conducting, anc stable at temperatures of evaporation and operation are suitable. Carbon layer 14 which is evaporated on the surface of silicon monoxide layer 12 is of various thick nesses depending upon the predetermined resistance value desired. Thus, after a predetermined resistance value i: selected for the resistor, the carbon layer is evaporate( onto layer 12 in an appropriate thickness to provide thi: resistance value for the device. Electrodes 13` are showt as being of aluminum. However, any electrically `conductive material which is stable at the evaporation tempera ture and the operating temperature is employable, Thus other materials such as niobium, tantalum, gold, chro` mium and copper are useable as electrode materials Other velectrode configurations are also suitable.

Layers 12 and 15 of silicon monoxide are shown a: of identical dimensions which do not cover the entire surface of su-bstrate 11. However, these layers may bt made of varying dimensions and are not limited to the dimensions shown with relationship to the substrate.

In FIGURE 3 of the drawing, apparatus is showr generally at 16 for `forming electrical resistors in accordance with my invention. A metal base 17 has z raised center portion 18 with an aperture 19 therein anc an outer rim 20 on which is positioned a rubber gaske4 21. A glass bell jar 22 is positioned on gasket 21 adjacen the edge of center portion 18 of base 17. An evacuatior line 23 is connected to aperture 19 and to a pump (no shown) to evacuate a cihamber 24 defined =by bell jar 22 and center portion 18 of base 17.

A centrally located insulating sleeve 25 is shown extending through center portion 18 of base member 17. Sleeve 25 positions therein a pivotal rod 26 with an arm 27 extending therefrom. Arm 27 supports a member 28 on the underside of which is provided a recess 29 to support an electrically non-conductive substrate 30 therein. Rod 26 is moved from outside chamber 24 by any suitable means, such as, an arm 31. Heating means 32 in the form of an electrically insulated coil imbedded ir member 28 or affixed to the lower surface of this member provides heating for substrate 30. Leads 33 from coil 32 extend through arm portion 27, and rod 26 to an appropriate power source (not shown) which has an adjustable power control (not shown). In addition to the pivotal action of member 28 by corresponding movement of handle 31 outside chamber 24, rod 26 may be raised and lowered by pressure exerted on handle 31.

A first pair of insulating sleeves 34 are positioned on center portion 18 of base member 17 and support therein a pair of electrically conductive rods 35. An electrically conductive boat 36, for example, of tantalum is supported at opposite ends of rods 35 by means of a pair of brackets 37. A quantity of silicon monoxide 38 in particle form is provided in boat 36 for subsequent evaporation. A pair of leads 39 and 40 connect opposite rods 35 to a power source 41 which is regulated by an adjustable power control 42. A second pair of electrically insulating sleeves 34 are positioned on center portion 18 and support therein a pair of electrically conductive rods 35. A similar pair of leads 39 and 40 are connected to a power source 41 which is regulated by an adjustable power control 42. A carbon rod 44 is supported in the side wall of one of rods 35 and extends outwardly therefrom. A pointed carbon rod 45 is mounted on the other electrically conductive rod 35 by means of a conventional spring loaded device 46 whereby the point of rod 35 is in contact with the end of carbon rod 44 thereby prod viding a carbon source for evaporation. A third pair of electrically insulating sleeves 34 are positioned on center portion 18 and support -therein a pair of T-shaped members 47. A similar circuit comprising a pair of leads 39 and 40 are connected to a power source 41 which is regulated by an adjustable power control 42. An electrically conductive wire coil 48, for example of tungsten, is connected between both opposite ends of T-shaped members 47. An aluminum evaporant in rod or other form is positioned within each coil 48.

A mask 49 with a rectangular opening 50 is shown positioned above boat 36 and supported in any suitable manner (not shown). A mask 51 with a smaller rectangular opening 52 is shown above carbon rods 44 and 45 and supported in any suitable manner (not shown). A third mask 53 with a pair of openings 54 is shown supported above T-shaped members 47 and wire coil 48 whereby openings 54 are in registry with wire coil 48. Mask 53 is supported in any suitable manner (not shown).

I discovered that an electrical resistor is formed on a substrate by positioning an electrically non-conductive substrate within a chamber, evacuating the chamber to a pressure range of 1 109 to 5x10*5 millimeters of mercury, heating the substrate to a temperature in a preferred temperature range of 200 C. to 600 C., evaporating carbon within the chamber and condensing a layer of carbon on the substrate, evaporating an electrically conductive metal within the chamber and condensing the metal as a pair of spaced apart electrodes on the carbon layer, and evaporating silicon monoxide within the chamber and condensing a layer of silicon monoxide on the carbon layer.

I discovered further that an electrical resistor could be formed on a substrate by employing the above steps but prior to evaporating carbon, silicon monoxide is evaporated within the chamber and condensed as a layer of silicon monoxide on the substrate whereby the subsequent evaporation of the carbon in the chamber provides condensation of a layer of carbon on the silicon monoxide layer.

An electrical reisstor is also formed on a substrate which includes either of the above series of steps and additionally the steps of providing a boron vapor within the chamber while the carbon is evaporated within the chamber thereby condensing a layer of carbon with two to ten weight percent of boron on the silicon monoxide layer or on the electrically non-conductive substrate.

I found that a suitable substrate, on which my improved electrical resistor is formed, must be of an electrically non-conducting material. For example, slide glass, Pyrex glass, or a glazed ceramic is suitable. Addiitonally, other materials which are characterized by being electrically non-conductive, and stable at temperatures of evaporation and operation are suitable for employment as substrates.

I found that an improved electrical resistor is formed in accordance with my invention by employing a chamber wherein a conventional pressure range of 1 109 to 5x10-5 millimeters of mercury is employed. I found further that it is necessary to heat the substrate during evaporation and condensation of the various layers. While the precise temperature of the substrate heating is not critical, I have found that a preferred temperature range from 200 C. to 600 C. provides a desirable range whereby the layers evaporated and condensed on the substrate adhere firmly to the substrate. I found that silicon monoxide can be evaporated Within the chamber and condensed as a layer of silicon monoxide on the substrate by employing various sources of silicon monoxide. For example, I prefer to employ a boat of an electrically conducting material which contains silicon monoxide particles and which boat is connected electrically to a power source to evaporate the silicon monoxide and subsequently condense the silicon monoxide substrate.

I found that the resistance value of the evaporated carbon layer is predetermined lby controlling the thickness of the carbon layer deposited on the silicon monoxide layer or on the substrate thereby providing a predetermined resistance value. A pair of carbon rods is shown as the preferred way to provide a carbon source which source is connected electrically to a power source to provide for evaporation of the carbon from the carbon rods onto the layer of silicon monoxide or onto the substrate. Various other suitable means of evaporating carbon for condensation are also employable in this process. Additionally, if it is desired the layer of carbon is deposited on the substrate or on the iirst layer of silicon monoxide on the substrate and thereafter the thickness of the carbon may be adjusted to provide a predetermined resistance value. For example, this is accomplished by providing a source of oxygen to the bell jar whereby the carbon condensed on the substrate or on the silicon monoxide layer is oxidized to the desired thickness to provide the predetermined resistance value. The carbon may be deposited directly on the substrate or it may be deposited on a rst layer of silicon monoxide which has been previously deposited on the substrate.

Additionally, boron is added with the carbon to provide a car-bon layer containing 2 to 10 weight percent of boron. This is accomplished in various suitable manners. For example, the carbon is evaporated from a carbon source while a boron vapor is provided within the cham- Iber during the carbon evaporation to condense a layer containing carbon and two to ten weight percent of boron on the substrate or on the silicon monoxide layer. A carbon compound containing a suflicient weight percent of boron to provide two to ten weight percent boron in the condensed layer of carbon on the substrate is also a suitable manner to accomplish the provision of a layer of carbon with the above percentage of boron. Other boron compounds are suitable within the system to provide the desired weight percent of boron within the carbon layer deposited on the substrate or the silicon monoxide layer.

I found that a number of materials are employable as electrode materials. For example, aluminum, niobium, tantalum, gold, chromium and copper are suitable as such electrode materials. While a particular electrode configuration is described above and shown in the drawing, other electrode configurations are sutable.

I found further that the resistor is formed in an operation whereby the vacuum is not broken. This is accomplished by employing masks within the evacuated chamber and moving the substrate from mask to mask to provide the desired layers in the proper sequence. If it is desired, the substrate is made stationary within the chamber and the mask and sources rotated.

In the operation of the apparatus shown in FIGURE 3 of the drawing to provide an improved electrical re,-

sistor in accordance with my invention, silicon monoxide particles 38 are provided in boat 36, carbon rods 44 and 45 are employed, and a pair of wire coils 48 of tungsten with an aluminum evaporant rod positioned in each of the coils are used. A substrate 30, such as, of Pyrex glass, is inserted in recess 29 of member 28. Bell jar 22 is positioned on rubber gasket 21 and its inner edge is adjacent to center portion 18 of lbase member 17. The pump (not shown) evacuates chamber 24 to a pressure in the pressure range of 1 10`9 to 5 105 millimeters of mercury. Substrate 30 is heated then to a temperature in the preferred temperature range of 200 C. to `600 C. by heating coil 32 contained within member 28.

Member 28 is pivoted =by means of handle 31, which is ,located outside of bell jar 22, to mask 49 and 4lowered so that substrate 30 is in registry with opening 51 in mask 49. First control 42 is operated thereby heating gradually silicon monoxide particles 38 lin boat 36 to cause these particles to evaporate. The silicon monoxide evaporates from particles 38 and condenses as a layer of silicon monoxide 12 on the surface of substrate 30. The heating -is then discontinued.

Handle 31 is moved to raise member 28 from mask 49 and to pivot member 28 to mask 51. Handle 31 is then depressed thereby lowering member 28 to mask 51 and providing the center portion of substrate 30 in registry with opening 52 of mask 51. Second control 42 is then operated thereby heating the carbon source of rods 44 and 45 and causing evaporation of carbon from the point of rod contact. This carbon which is evaporated condenses as a layer of carbon 13 on the silicon monoxide layer. The thickness of this layer is controlled to provide a predetermined resistance value for the resistor. The heating is then discontinued.

Handle 31 is moved upward to raise member 28 and is moved to pivot member 28 to mask 53. Member 28 is then lowered to be in contact with or in close relationship to mask 53 whereby two spaced apart portions of carbon layer 13 of the coated substrate are in registry with the pair of spaced apart openings 54 in mask 53. Third control 42 is then operated thereby heating tungsten wire coils 48 and causing an evaporation of the electrically conductive aluminum evaporant positioned therein which aluminum condenses as a pair of spaced apart electrodes 14 on the carbon layer 13. The heating is then discontinued.

Handle 31 is then employed again to raise member 28 away from mask 53 and move member 28 to the mask 49. Member 28 is then lowered to contact or be in close relationship to mask 49 whereupon carbon layer 13 and a portion of both electrodes 14 of the coated substrate 30 are in yregistry with opening 50 in mask 49. First control 42 is operated aga-in to heat up gradually the silicon monoxide particles 38 in boat 36 to evaporate additional silicon monoxide which condenses as a second layer of silicon monoxide on the carbon layer 13 and a portion of both electrodes 14. The heating is then discontinued.

Member 28 is then raised from mask 49. The substrate heating provided by coil 32 is then discontinued. The apparatus is allowed to cool to room temperature within chamber 24. Chamber 24 is then returned to atmospheric pressure whereupon bell jar 22 is removed. Substrate 30 with the carbon resistor formed thereon is then easily removed lby sliding substrate 30 from recess 29 on member 28.

In the above operation of this device, an electrical resistor is also formed by depositing a first layer of carbon on the substrate. A pair of spaced apart electrodes are then condensed on the carbon layer. A layer of silicon monoxide is then condensed on the carbon layer and covers partially the electrodes. Furthermore, in the operation of this device, a source of boron such as provided in a carbon-boron compound is substituted for the carbon rods 44 and 45, a separate boron vapor is provided within the chamber, or a separate boron compound is positioned adjacent to and heated separately during the evaporation of the carbon whereby a carbon layer containing two to ten weight percent of boron is deposited either directly on the substrate or on the silicon monoxide layer. While it is not shown in the drawing, a separate source of oxygen can be provided to chamber 24 after the carbon layer is deposited directly on the substrate or on the silicon monoxide layer to etch the carbon layer to the correct thickness for a predetermined resistance value for the carbon resistor.

Several examples of electrical resistors which were made in accordance with the present invention are as follows:

In the first four examples, apparatus was set up in accordance with FIGURE 3 of the drawing to form electrical resistors in accordance with my invention. In each of these examples, silicon monoxide particles were employed in an electrically conductive boat, a pair of carbon rods were employed for the carbon source, and tungsten wire coils with an aluminum evaporant were employed for the source of metal for the electrodes. In each example, a Pyrex glass substrate was positioned within the chamber after which the bell jar was placed on the rubber gasket. The pump (not shown) evacuated the chamber to a pressure of 2 106 millimeters of mercury. The substrate was heated by the substrate heater to a temperature of 400 C. Silicon monoxide was evaporated from particles through a mask and condensed as a layer of about 2,000 A. thick on the substrate. A layer of carbon was then evaporated from the carbon rods through a mask and deposited as a layer on the silicon monoxide layer. In each of the four examples, the thickness of the carbon varied but the thickness was less than 1000 A. thick for each four layers. During this second evaporation, the temperature of the substrate was raised from 400 C. to 450 C. The substrate temperature was then lowered to 200 C. during the evaporation of the aluminum through a mask and condensation as a pair of spaced apart electrodes on the carbon layer. A second layer of silicon monoxide was then evaporated from particles through a mask and condensed on the carbon layer to provide a layer of about 3000 A. thickness while the substrate temperature was maintained at about 300 C. Subsequently, substrate heating was discontinued and the device allowed to cool to room temperature. The chamber was returned to atmospheric pressure after which the bell jar was removed.

The electrical resistors formed by this method each provided an electrically non-conducting substrate, a layer of silicon monoxide adhering rmly to the substrate, a layer of carbon with a predetermined resistance value adhering to the silicon monoxide layer, a pair of spaced apart electrically conductive electrodes adhering to the carbon layer, and a second layer of silicon monoxide adhering to the carbon layer. These resistors exhibited temperature coetiicient of resistivity values from -400 p.p.m./degree C. to -700 ppm/degree C. from room temperature to C.

While each of the above resistors had a carbon layer less than 1000 A. thick, this thickness vari-ed from Example 1, lwhich had the thinnest layer, to Example 4, which had the thickest layer. These examples are set forth below in Table I which recites the resistance in ohms per square of each resistor; the percentage of permanent resistance change after a ten minute heat treatment at 400 C., and the percentage of permanent resistance change after a 175 hour heat treatment at 150 C.

TABLE I Example Resistance, AR, 400 C., AR, 150 C., Number Ohms/El percent percent In Examples -7, apparatus was set up in accordance with FIGURE 3 of the drawing to form electrical resistors in accordance with my invention. In each of these examples, silicon monoxide particles were employed in an electrically conductive boat, a pair of carbon rods were employed for the carbon source, and tungsten wire coils with an aluminum evaporant were employed for the source of metal for the electrodes. In each example, a glazed ceramic substrate was positioned within the charnber after which the bell jar was placed on the rubber gasket. The pump (not shown) evacuated the chamber to a pressure of 2 10d6 millimeters of mercury. The sub strate was heated by the substrate heater to a temperature of about 300 C. A layer of carbon was evaporated from the carbon rods through a mask and deposited as a layer less than 1000 A. thick on the substrate. The substrate temperature was then lowered to 200 C. during the evaporation of the aluminum through a mask and condensation as a pair of spaced apart electrodes on the carbon layer. A layer of silicon monoxide was then evaporated from particles and through a mask and condensed onto the carbon layer to provide a layer of about 3000 A. thickness while the substrate temperature was maintained at about 300 C. Subsequently, substrate heating was discontinued and the device allowed to cool to room temperature. The chamber was returned to atmospheric pressure after which the bell jar was removed.

The electrical resistors formed by this method each provided an electrically non-conducting substrate, a layer of carbon with a predetermined resistance value adhering firmly to the substrate, a pair of spaced apart electrically conductive electrodes adhering to the carbon layer, and a layer of silicon monoxide adhering to the carbon layer. These resistors exhibited temperature coefiicient of resistivity values from -400 ppm/degree C. to -700 ppm/degree C. from room temperature to 150 C.

Examples 5-7 are set forth below in Table II which recites the resistance in ohms per square of each resistor; the percentage of permanent resistance change after a ten minute heat treatment at 400 C., and the percentage of permanent resistance change after several hundred hours heat treatment at 150 C.

While other modifications of this invention and variations of method which may be employed within the scope of the invention have not been described, the invention is intended to include such that may be embraced within the following claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A method of forming an electrical resistor which comprises positioning an electrically non-conductive substrate within a chamber, evacuating said chamber to a pressure range of 1x10-9 to 5 l0*5 mm. of mercury, heating said substrate, evaporating silicon monoxide within said chamber and condensing a layer of silicon monoxide on said substrate, evaporating carbon within said chamber and condensing a layer of carbon on said silicon monoxide layer, evaporating an electrically conductive metal within said chamber and condensing said metal as a pair of spaced apart electrodes on said carbon layer, oxidizing a portion of said carbon layer and subsequently evaporating silicon monoxide within said chamber and condensing a second layer of silicon monoxide on said carbon layer.

2. A method of forming an electrical resistor which comprises positioning an electrically non-conductive substrate within a chamber, evacuating said chamber to a pressure range of 1 l09 to 5 105 mm. of mercury, heating said substrate, evaporating silicon monoxide within said chamber and condensing a layer of silicon monoxide on said substrate, evaporating carbon within said chamber while simultaneously providing a boron vapor within the chamber to condense a layer of carbon having 2-10% boron content atop the silicon monoxide layer, evaporating an electrically conductive metal within said chamber and condensing said metal as a pair of spaced apart electrodes on said boron containing carbon layer, and evaporating silicon monoxide within said chamber and condensing a second layer of silicon monoxide on said boron containing carbon layer.

3. A method of forming an electrical resistor which comprises positioning an electrically non-conductive substrate within a chamber, evacuating said chamber to a pressure range of 1 10r9 to 5 105 mm. of mercury, heating said substrate, evaporating carbon Within said chamber and condensing a layer of carbon on said substrate, evaporating an electrically conductive metal within said chamber and condensing said metal as a pair of spaced apart electrodes of said carbon layer, oxidizing a portion of said carbon layer, and subsequently evaporating silicon monoxide within said chamber and condensing a layer of silicon monoxide on said carbon layer.

4. A method of forming an electrical resistor which comprises positioning an electrically non-conductive substrate within a chamber, evacuating said chamber to a pressure range of 1 109 to 5 10-5 mm. of mercury, heating said substrate, evaporating carbon within said chamber while simultaneously providing a boron vapor within the chamber and condensing a layer of carbon having a 2-10% boron content atop said substrate, evaporating an electrically conductive metal within said chamber and condensing said metal as a pair of spaced apart electrodes on said boron containing carbon layer, and evaporating silicon monoxide within said chamber and condensing a layer of silicon monoxide on said boron containing carbon layer.

References Cited UNITED STATES PATENTS 2,635,994 4/1953 Tierman 117-226 X 2,671,735 3/1954 Grisdale et al. 117-226 2,764,510 9/1956 Ziegler 117-216 2,849,583 8/1958 Pritikin 117-2l7 X 2,927,048 3/1960 Pritikin 117-216 X 3,308,528 3/1967 Bullard et al.

OTHER REFERENCES Lessor, A. E., and Thun, R. E., Multilayered Thin Films, in IBM Technical Disclosure Bulletin 5 (4), September 1962.

RALPH S. KENDALL, Prima/y Examiner.

C. K. WEIFFENBACH, Assistant Examiner.

U.S. Cl. X.R. 

